Friday, February 15, 2013

The Chelyabinsk Event, 15 February 2013

Early today a huge aerial explosion rocked the Siberian city
of Chelyabinsk, collapsing or damaging buildings and shattering windows throughout
the city.Slivers of window glass
accelerated by the blast wave from the explosion sent at least 500 people to
hospitals for treatment, with many more injured less severely.The media are trumpeting a “meteor” explosion
and speculating about a link to this afternoon’s flyby of the Near-Earth
Asteroid 2012 DA14.I am being barraged
with requests for information, even though the amount of solid quantitative
date now available is minimal.Nonetheless, there are several points that can confidently be made.

1.This was
not a meteor.A meteor is an optical
phenomenon, a flash of light seen in the sky when a piece of cosmic debris (usually
dust- or sand grain-sized) enters Earth’s upper atmosphere, converts its huge
kinetic energy into heat, and “burns up” (vaporizes), usually at an altitude of
at least 100 km.The Chelyabinsk object
was a fragment of asteroidal or cometary origin, probably several meters in
diameter, properly called a “meteoroid” or, more loosely, a “small asteroid”.A brilliant fireball seen in the atmosphere
is called a bolide.Some bolides, caused
by entry of large pieces of hard rock, drop meteorites on the ground: a meteorite
is a rock of cosmic origin that reaches the ground in macroscopic pieces (not
dust or vapor).Some bolides are
cometary fluff, of which nothing is strong enough to survive as a meteorite.This body was fairly strong, and is therefore
more likely to be an asteroid-derived meteoroid.Indeed, some Russian sources are claiming
that a meteorite from the blast fell in a lake in nearby Chebarkul, Russia, but
this has not been verified.Such judgments
are tricky because the distance to the fireball is usually wildly
underestimated (“it cleared my barn, so it must have been at least 50 feet up”).

2.The path of 2012 DA14 is well understood.It is in a generally Earth-like orbit, except
that its orbit is inclined relative to the plane of Earth’s orbit around the
Sun.To first approximation, it is
neither “catching up with Earth” or “being swept up from behind by Earth”: Its
motion relative to Earth it basically at right angles to the direction of our
orbital motion.It will pass us from
south to north.Think of two cars on the
freeway traveling in the same direction at the same speed, one of them in lane
2 and the other switching from lane 1 to lane 3.Chelyabinsk is basically “behind the Earth”
as seen by the approaching asteroid.In
other words, the Chelyabinsk object is not associated with 2012 DA14.

3.There is also speculation about 2012 DA14 being
accompanied by debris and even small satellites.This is well founded, but these fragments, produced
by collisions of small rocks with the asteroid, must follow paths that are
closely similar to that of the parent asteroid.If they exist, and if they hit Earth, they will do so near or to the
south of the Equator.Incidentally, the
orbits of satellites of NEAs are usually close in, simply because distant
satellites will be stripped away by the tidal forces of the Sun (and now,
during a close flyby, by Earth also), and their orbital speeds are tiny
(centimeters to meters per second).

4.There was an early report of Russia scrambling
jet fighters to intercept the object.Here’s how that works: suppose the bolide is traveling at the absolute
minimum entry speed of about 10 km/second and radar picks it up at a range of
1000 km.This radar detection tells them
the speed of the bolide. From detection
to arrival they have 100 seconds, tops.Then they have the interesting task of intercepting something moving 10 (or
20) km/s with an airplane that has a top speed of, say, Mach 2.5.That’s about 0.75 km/s.See the problem?The real military significance of impact
airbursts is not that it is impossible to intercept them with jet aircraft: it
is the danger of a completely unpredicted high-yield aerial explosion occurring
over a major city in a heavily armed, politically unstable region: think, Tel
Aviv, Tehran, etc. Instant World War
III.

5.There’s a lot of talk and speculation about how
rare such events are.Any meaningful
statistics would require that we know how big it really was (the bigger the
rarer).But a reasonable first guess is that
this is a decadal object: ten per century hitting Earth, of which typically
nine are in sparsely populated or unpopulated areas, such as the Tunguska Event
of 30 June 1908 and the two Brazilian events around 1930.We’ll know more about the size and blast
energy soon. So my take is that these
events are not rare, but having one over a city is unusual.

In the
1997 edition of my book Rain of Iron and
Ice I included a lengthy table of reports from public media and scientific
journals documenting injuries, deaths, property damage, and near-misses due to
cosmic impact events, ranging from a meteorite knocking off a girl’s hat to a powerful
airburst showering a city in China with tens of thousands of stones and killing
over 10,000 people [Ch’ing-yang, Shansi, 1490 AD; source: Kevin Yao, Paul
Weissman, and Don Yeomans, Meteoritics29, 864-971 (1994)].My Monte Carlo models of the long-term
effect of impact events in my 2000 book Comet
and Asteroid Impact Hazards on a Populated Earth provide quantitative estimates
of the events occurring in hundreds of 100-year computer models.In it, Model H89 generates a low-altitude
airburst of 83 megatons yield at an altitude of 19 km.A random location generator placed this blast
over the city of Orleans, France, killing 40,000 people and igniting a
firestorm.After this model was published,
Pete Worden, who was then Commandant of Falcon AFB in Colorado Springs, sent me
an account that he had found in Bishop Gregory of Tours’ History of the Franks: “580
ADIn Louraine, one morning before the
dawning of the day, a great light was seen crossing the heavens, falling toward
the east.A sound like a tree crashing
down was heard over all the countryside, but it could surely not have been any
tree, since it was heard more than fifty miles away… The city of Bordeaux was
badly shaken by an earthquake… The city of Orleans also burned with so great a
fire that even the rich lost almost everything.”

9 comments:

Dr Lewis. Would you generally agree with my overall assessment that it is only the asteroids between 20-30m that pose a risk to human life?

Below 20m and they are not powerful enough to kill people on the ground. Greater than 30m and we are now routinely getting the more than two days' warning necessary to evacuate a city. Greater than say 500m and we are getting into sizeable percentages of those known years before they would likely impact. And those greater than 10km we know that none are on an impact trajectory because our telescopes are sensitive enough that we know about 100% of them.

If so, then we could focus on closing the 20-30m window by implementing technology aimed towards detecting them while they are incoming.

There were also some reports that the Russians had fired anti-aircraft missiles (AAMs) t the inbound object. This poses the same problem as intercepting it with aircraft - that of warning. With AAMs, you have to be on alert, with Rules of Engagement that allow you to fire upon warning, a very itchy trigger finger, and then have the flightpath within the range of the launch site. No internal excitement that I know of in Southern Russia at the time that would lead to that sort of high alert status, so no missile launch. Additionally, most of the larger AAMs will have some sort of contrail, mostly due to the exhaust plume of the propulsion system. Best to you and yours. Cheers -

In the long run, the overwhelming majority of fatalities are caused by the one or two most severe events. The size of that even depends on the size of the time intercal we wish to consider. But it is also true that the overwhelming majority of lethal events are quite small, involving sub-megaton explosions and a handful of deaths. If we are concerned only about the 1000-year prospect, average fatality rates are about 100 people per year, with the largest single event accounting for about 70 to 90% of the deaths. Typically there are about 20 fatal events per millennium, mostly small (< 1 Mt) explosions with few fatalities. These results are spelled out in considerable detail in my book, "Comet and Asteroid Impact Hazards on a Populated Earth". Note that the longer a view you take, the higher the averaged annual death toll is!John S. Lewis

Dr. Lewis. Does that analysis take into account detection and evacuation? Although the larger ones may put more lives at risk, it seems unlikely to me that they would go undetected and, once known that people wouldn't evacuate the area. If this is the case then it would help us put more resources towards detecting the 20-30m incomings.

Over time scales comparable to the lifetimes of civilizations (several thousand years) the largest impactor will be in the 100-meter class and deliver 50-100 Megatons of explosive energy. Our population statistics are pretty good for this size range, but the degree of completeness is poor. Most bodies of this that hit Earth would do so with little or no advance warning, so evacuation would not be an option. Cataloging all the 100-m bodies is a very long and expensive project because there are several hundred thousand such bodiesin the NEA population.

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Chelyabinsk wasn't the only fireball reported on the same day thatDA14 passed by. I don't know if I believe Frank Davis that most ofthem actually were related to DA14 in some way, but he has acouple possible explanations for the different trajectories. Andthese are supported by his computer models. If there's a chancethat he's right it means that near-miss asteroids can beaccompanied by such events -- meaning a serious risk whenApophis passes in 2029. If he's wrong, I'd like to see NASAor someone specifically debunk his models.

John S. Lewis

John S. Lewis is a professor emeritus of planetary science at the University of Arizona’s Lunar and Planetary Laboratory and is Chief Scientist at Deep Space Industries. His interests in the chemistry and formation of the solar system and the economic development of space have made him a leading proponent of turning potentially hazardous near-Earth objects into lucrative space resources. Prior to joining the University of Arizona, Lewis taught space sciences and cosmochemistry at the Massachusetts Institute of Technology. He received his education at Princeton University, Dartmouth College and the University of California, San Diego, where he studied under Nobel Laureate Harold Urey. An expert on the composition and chemistry of asteroids and comets, Lewis has written such popular science books as "Mining the Sky", "Rain of Iron and Ice", and "Worlds without End".